Yep. You got it.
However the locos will need to be evenly matched or the throttle might "turtle" one and "rabbit" the other.
You could use a few diodes to even them up some. Each eats about .75v. in series (inline) . Ive made throttle control for constant power supplies that way. Put them on a switch and you could swap which train goes on which line.
So you can reverse, you need a pair, bands opposed, legs connected. Each will handle the opposing dc+ or dc- while the other does nothing. The pairs can then be stacked in series with the same drop rate as singles.
One diode limits you to one polarity setting. Reverse toggle will then only work on one setting.
A bridge rectifier is four diodes in one package, arranged to make dc from ac. But they can be wired to give your dc a 1.5v drop vs two in series if you like the packaging or price. (all are cheap at a supply house)
Another consideration is how big a draw two locos are, vs how big your powerpack is. You want the supply to push about 20% more than the trains use. (in amps,voltamps,watts, etc) and don't forget to add power used by passenger cars, track accessories (lamps, motors etc)
